Cream cheese compositions are generally high fat products obtained as the curds from cream cheese type lactic fermentations. They contain about 55% moisture, 32-34% fat, 5-7% protein, and 2-3% lactose. The consistency of these compositions, permitting spreadability while retaining firmness, is modulated by the addition of vegetable gums to them. Cream cheeses are mildly acidic, having pH values around 4.5 to 5.0. Their delicate buttery flavor is generally ascribed to the presence of volatile organic flavoring compounds such as diacetyl. The preparation of cream cheese compositions includes culturing a cream cheese fermentation mix to a pH of about 4.5 to 4.6, exposing the result to temperatures of about 180.degree. F. for a brief time, centrifuging to separate the cream cheese curd from the whey, blending the hot curd, at about 175.degree. F., with a dry blend of vegetable gum and salt, and cooling prior to packaging.
A difficulty with present cream cheese products is that, although they are generally stable and have an extended storage life at refrigerated temperature, they tend to lose flavor and develop bacterial or mold growth upon prolonged storage. The microbiological contaminants may, in general, include both bacteria, such as various examples from the genus Bacillus, various mesophilic and thermophilic cocci, and a variety of molds and yeasts. A variety of possible origins for these contaminants exists. They may originate in the ingredients used in the preparation of the cream cheeses, and be such that they survive the elevated temperatures used in the manufacturing process. They may be airborne or surface borne in the facilities used in manufacture. Additionally they may be present on the containers and wrappings used in packaging the final product for shipment and sale.
Nisin is a peptide-like antibacterial substance produced by microorganisms such as Lactococcus lactis subsp. lactis (formerly known as Streptococcus lactis). Its structure is illustrated in U.S. Pat. No. 5,527,505 to Yamauchi et al. The highest activity preparations of nisin contain about 40 million IU per gram. A commercial preparation, NISAPLIN.RTM., containing about 1 million IU per gram is available from Aplin & Barrett Ltd., Trowbridge, England. Nisin has no known toxic effects in humans. It is widely used in a variety of prepared dairy foods. Experimental use in preserving other foods has also been reported. The cultures that produce nisin, being lactic fermentations, generally produce lactate as well.
A number of efforts have been reported since 1975 directed to reducing uncoupled acid production in dairy fermentations by controlling the post-fermentation acidification of yogurt. In some of these studies, a nisin producing culture was introduced in an attempt to inhibit these effects. Kalra et al. (Indian Journal of Dairy Science 28: 71-72 (1975)) incorporated the nisin producing culture Streptococcus lactis (now known as L. lactis subsp. lactis) along with the yogurt culture before fermentation. Others introduced nisin in milk prior to fermentation (Bayoumi, Chem. mikrobiol. technol. lebensm. 13:65-69 (1991)) or following fermentation (Gupta et al., Cultured Dairy Products Journal 23: 17-18 (1988); Gupta et al., Cultured Dairy Products Journal 23: 9-10 (1989)). In all cases, the rate of post-fermentation acidification was only partially inhibited by these treatments and the yogurt continued to become more acidic throughout its shelf life.
In U.S. Pat. No. 5,527,505, by Yamauchi et al., yogurt was produced from raw milk by incorporating a nisin-producing strain, Lactococcus lactis subsp. lactis, along with the traditional yogurt culture consisting of Streptococcus salivarius subsp. thermophilus (ST) and Lactobacillus delbrueckii subsp. bulgaricus (LB). Yamauchi et al. teach that the lactococci are needed to secrete the nisin, whose effect is to retard the activity of ST and LB. The resulting yogurt therefore contains the lactococci used to produce the nisin. Nonetheless, the acidity of yogurt containing the nisin-producing bacteria increased by 64% to 96% in 14 days, in various experiments inoculated with differing amounts of L. lactis subsp. lactis, compared to the initial acidity at the completion of fermentation. Other studies (Hogarty et al., J. Food Protection 45:1208-1211 (1982); Sadovski et al., XX International Dairy Congress, Vol. E: 542-5-44 (1978)) also noted acid production and development of bitterness at low temperature by some mesophilic starter lactococci in dairy products.
In U.S. Pat. No. 5,015,487 to Collison et al., the use of nisin, as a representative of the class of lanthionine bacteriocins, to control undesirable microorganisms in heat processed meats is disclosed. In tests involving dipping frankfurters in nisin solutions, the growth of L. monocytogenes was effectively inhibited upon storage at 4.degree. C.
Chung et al. (Appl. Envir. Microbiol. 55, 1329-1333 (1989)) report that nisin has an inhibitory effect on gram-positive bacteria, such as L. monocytogenes, Staphylococcus aureus and Streptococcus lactis, but has no such effect on gram-negative bacteria such as Serratia marcescens, Salmonella typhimurium and Pseudomonas aeruginosa when these microorganisms are attached to meat.
Nisin has been added to cheeses to inhibit toxin production by Clostridium botulinum (U.S. Pat. No. 4,584,199 to Taylor). U.S. Pat. No. 4,597,972 to Taylor discloses a detailed example in which chicken frankfurter components are shown to require the presence of both added nitrite and added nisin in order to prevent or delay botulinum toxin production when challenged with C. botulinum.
In U.S. Pat. Nos. 4,888,191 and 5,017,391, Anders et al. disclose compositions and methods related to the use of lactate salts to delay C. botulinum growth in a foodstuff such as fish or poultry. The foods are heated to a temperature sufficient to cook the meat but not to sterilize the product. Anders et al. suggest that lactate may be used alone, or in combination with other agents such as sodium nitrite. These patents fail to discuss nisin or its properties.
Maas et al. (Appl. Envir. Microbiol. 55, 2226-2229 (1989)) report that lactate, when incorporated into a turkey meat vacuum-packed composition, delays the generation of botulinum toxin in a manner directly dependent on the concentration of lactate introduced into the composition. Maas et al. do not mention nisin.
As shown herein, commercially produced cream cheese compositions are readily contaminated by microorganisms. Accordingly, there is a need for cream cheese compositions and methods of producing them that inhibit the growth of objectionable and pathogenic microorganisms, using natural or innocuous ingredients. In particular, there is a need for such compositions and methods that avoid the introduction of artificial or synthetic preservatives, the use of which has led sectors of the consuming public to avoid purchasing products containing such preservatives. The present invention addresses this need.